PLTL Workshop – Assessment (2)
CHEM 111
“Self-assessment is universal truth”
- Anonymous
Chemistry 111
Final Assessment
T
he following assessment is based on the workshops you have encountered this semester.
To better develop the materials and to make sure that you and future students are wellinstructed, we have developed this set of follow-up exercises to be done WITHOUT the
assistance of the peer leader or the other members of your group. Though this assessment is
being done on an individual basis, you will not be graded on these exercise but should instead
use this exercise to determine how much you really understood from your earlier workshops.
Also, we have included two assessment surveys for you to give feedback about the
quality of your instruction, your peer-to-peer interactions, your relationship with your peer
leader as well as your opinions about your progress in the overall course. Though your peer
leaders will go over your answers to the exercises, your instructors and peer leaders will NOT
have access to your surveys/comments, so please feel free to be as forthcoming as possible.
Chemical Reactions
Chemical reactions have been known and applied for many years. For example, ancient people
knew that if carbon (charcoal) was mixed with iron oxides and heated, elemental iron and
carbon dioxide would be formed. This practical knowledge was attained without any concept of
atoms, molecules and reactions. By the nineteenth century, the study of stoichiometry allowed
chemists to determine masses of reactants and products during the reaction. We will use
stoichiometry to allow us to predict the mass of a reactant or product using a balanced equation.
Balanced Equations
Chemical equations may take many forms. The statement above that iron oxide reacts with
charcoal to produce elemental iron and carbon dioxide is one form of describing a chemical
reaction. However, this type of statement does not allow any quantitative information to be
determined. Using the information above, and a little knowledge of chemical formulas, we can
produce a balanced chemical equation to use in quantitative determinations. The term “iron
oxide” is ambiguous because iron and oxygen may react with several different ratios. For our
purposes, we will assume that the iron oxide in question is iron(III) oxide. The formula of
iron(III) oxide is Fe2O3. Carbon is a monatomic element with the symbol C. Elemental iron is
Fe. Carbon dioxide has the formula CO2. We can write an unbalanced equation as:
Fe2O3 + C → Fe + CO2
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PLTL Workshop – Assessment (2)
CHEM 111
In order to get quantitative information from this equation it must be balanced. Balancing the
equation would result in the following equation:
2Fe2O3+ 3C → 4Fe + 3CO2
This equation tells us that 2 moles of Fe2O3 will react with 3 moles of C to produce 4 moles of
Fe and 3 moles of CO2. We will use this information to form conversion factors to allow us to
calculate the amount of one substance from the amount of another substance.
Molar Masses
An additional conversion factor necessary in stoichiometric calculations is molar masses. As
we learned in the last workshop, the atomic masses from the periodic table expressed in grams
represent Avogadro’s number of molecules. One mole is defined to be Avagadro’s number of
particles. If the atomic mass of an element is expressed in grams, this represents one mole of
atoms.
1 mol = 6.022x1023 atoms = atomic mass in grams
Similarly, the molar mass of substances is defined to be the mass of one mole of substance. For
compounds, calculation of the molar mass is accomplished by adding the atomic masses. If the
molar mass is expressed in grams, this represents one mole of the compound.
1 mol = 6.022x1023 formula units = molar mass in grams
We use the more general term “formula units” instead of molecules because ionic compounds
do not exist as molecules.
Factor Unit Conversions and Calculation of Moles
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PLTL Workshop – Assessment (2)
CHEM 111
We can use the conversion factors above to calculate the number of moles of a substance from
the mass of the substance.
mass of A x
1 mole of A
=
moles of A
molar mass of A
.
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PLTL Workshop – Assessment (2)
CHEM 111
Self Test
1. 1. A balloon is filled with helium gas at 27°C and 1.00 atm pressure. As the
balloon rises, the volume of the balloon increases by a factor of 1.60 and the
temperature decreases to 15°C. Assuming that no helium escapes from the
balloon, what is the final pressure?
2. Pyrophosphoric acid is composed of 2.27% hydrogen, 34.80% phosphorus, and
the remainder is oxygen.
a) What is the mass of each element present in a 125.0 g sample? What is the
number of moles present in the sample?
b) Find the simplest whole-number ratio of the number of moles of each
element. This can be found by dividing the number of moles of each element by
the number of moles of the element with the smallest number of moles. Your
result gives the empirical formula of pyrophosphoric acid.
c) The molar mass of pyrophosphoric acid is 177.97 g/mol. What is its
molecular formula?
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PLTL Workshop – Assessment (2)
3.
CHEM 111
Determine the freezing and boiling points of a solution made by dissolving 30.0
g of sucrose, C12H22O11, in 100.0 g of water.
4. Fill in the chart below, using a periodic table as the only source of additional
information.
Compound
Empirical Formula
Molar Mass
Moles in 100.0 g
H2O
H2O2
BaSO4
5.
Calculate the percentage composition by mass of each of the compounds in
Question 1. The first compound has been completed as an example.
Compound
Percentage Composition by Mass
H2O
88.8% O, 11.2% H
H2O2
BaSO4
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PLTL Workshop – Assessment (2)
CHEM 111
Recitation Day and Time _________________________________________________
For each item, please circle the number that corresponds to your response:
1= strongly agree; 2= agree; 3= disagree; 4= strongly disagree
The Workshops are closely related to the material taught in the
1
2
3
4
1
2
3
4
I believe that the Workshops have improved my grade.
1
2
3
4
Interacting with the other group members increases my understanding.
1
2
3
4
The materials in the workshop were useful for group work.
1
2
3
4
others to benefit from the workshops.
1
2
3
4
I felt comfortable with the Workshop leader.
1
2
3
4
lectures.
Students who are uninterested or unmotivated made it difficult for
I would like to be a Workshop leader in the future.
On average, I spend the following number of hours per week studying (in addition to time spent
at lectures and Workshops):
a. 0-2 hours
b. 2-4 hours
c. 4-6 hours
d. 6-8 hours
e. 8-10 hours
Please rate each of the following activities according to the amount of Workshop time devoted
to the specified activity. Use the following scale:
1= most of the time; 2= a moderate amount of time; 3= a small amount of time; 4= almost
no time
The self test
1
2
3
4
Group activities
1
2
3
4
Small group problem solving
1
2
3
4
Independent problem solving
1
2
3
4
Laboratory connections
1
2
3
4
What was the last topic that you covered in lecture?
I. Workshop materials
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PLTL Workshop – Assessment (2)
CHEM 111
Did the workshops help learn the course material?
Did the group activities increase your understanding?
What part of the workshop materials was the most helpful? (Background
information, self-test, group problems, laboratory connections)
What part was the least helpful?
Are the problems too hard, too easy, or about right?
II. Role of the leader
What was the role of the leader in the group?
Was the workshop leader well prepared?
Did the workshops improve slightly, greatly or not at all since the first assessment?
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PLTL Workshop – Assessment (2)
CHEM 111
III. Group Interactions
How did you spend most of your time during the workshops?
Did you feel comfortable interacting with the group?
Did you learn from the other members in the groups?
What was the best part of the workshop?
What was the worst aspect of the workshop?
Do you feel confident that you will successfully complete Chemistry 111 with a
passing grade (C or better)?
IV. General observations and comments.
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